Various existing transmitters, and/or transceivers (herein after transmitter(s)) may employ a dual conversion architecture for up or down conversion of radio frequency (“RF”) signals. Such a transmitter is generally configured in the prior art to use two, separate local oscillators. More particularly, when the transmitter is a tunable transmitter, the dual conversion architecture comprises two or more separate local oscillators.
With reference to FIG. 1A, a prior art tunable dual conversion transmitter 100 comprises at least two local oscillators 110, 111. Local oscillator 110 is a tunable LO, that is tunable from 9.8 to 11.2 GHz. Tunable dual conversion transmitter 100 further comprises a first mixer 120, a filter 125, and a second mixer 130.
With reference to FIG. 1B, a prior art tunable transmitter 100′ comprises a local oscillator 110. Local oscillator 110 is a tunable LO, that is tunable from 10.133 to 10.6 GHz. Tunable transmitter 100′ further comprises a first mixer 120, and a filter 125. In this embodiment, spurious signals can be difficult to filter because IF related spurious signals are typically a small percentage away from generally desired RF passbands. Additionally, each band will require filtering to remove undesired IF related spurious signals. Thus, resulting in unnecessarily high material costs. This approach achieves a high phase noise degradation of approximately 20 log(3)=9.54 dB.
Use of two or more local oscillators in tunable transmitters that have dual conversion architecture has a number of disadvantages. For example, in such embodiments the Intermediate Frequency (“IF”) band following the first upconversion, at first mixer 120, must be just as wide as the final output band. Also, one of the oscillators, e.g. local oscillator 110, must be able to tune the transmitter across the entire band. Furthermore, the final upconversion uses a local oscillator at a high frequency that is more difficult to design with adequate phase noise than the first LO.
Wider filter passbands employed by such prior art may be difficult if not impossible to design because, in some instances, undesired higher order spurious mixer outputs related to multiples of LO and IF input frequencies may be too close to the pass-band to adequately filter or may even be inside the filter pass-band thus prohibiting a single filter approach. In any event, it may be difficult to design a cost effective device comprising multiple local oscillators in tunable transmitters with a dual conversion architecture. In some instances, use of such prior art designs may also necessitate use of a switchable bank of filters, which increases overall design cost. Use of prior art architecture also renders it difficult if not practically impossible to use highly integrated GaAs, silicon-germanium (SiGe) or other semiconductor processes in tunable transmitters and obtain sufficient spurious rejection, due to process variations that give rise to additional design limitations.
Thus, it is desirable to have a tunable transmitter with dual conversion architecture that overcomes some of these drawbacks of the prior art. It is further desirable to have a tunable transmitter with dual conversion architecture with lower material costs, that is easier to manufacture, that has improved phase noise characteristics, that reduces the range the LO has to tune, and that reduces the IF bandwidth.